Iron ore is destined to retreat back below $US50 a tonne next year as supplies go on rising, according to the top forecaster, who warned that weakening prices will probably encourage the sale of inventories.

The raw material will drop to average $US62 in the third quarter and $US59 in the final three months of this year before falling through 2018 to a low of $US41, said Justin Smirk, senior economist of Westpac Banking Corp. Westpac placed first in predicting prices in the first quarter, according to data compiled by Bloomberg.

Iron ore was whipsawed last week after hitting a near six-month low as investors weighed signals of strength in the largest user China, including steel output at a record in March, against prospects for rising supply. Top miners including Brazil’s Vale are bringing on new capacity, bolstering seaborne sales, at the same time that miners in China have been reviving production. Smirk said that there’d been a huge ramp-up in Chinese supply.

“As supply builds up and prices come off, people will begin to question the wisdom of holding on to inventories,” Smirk said in a phone interview on Friday. “The signs are now pushing in one direction: while we’ll get some volatility, the momentum is just on a downward trend now.”

Spot ore with 62 per ent in Qingdao fell 2.5 per cent to $US66.53 a tonne on Monday following a volatile week, according to Metal Bulletin. The commodity – which hit $US94.86 in February – averaged $US86 in the first three months of 2017 and is averaging $US72.32 so far this quarter. Futures in Dalian and Singapore fell, with the SGX AsiaClear contract as much as 3.9 per cent lower.